Grounding device for mounting strap

ABSTRACT

Grounding connection between the mounting strap of a wiring device and the metal housing in which the device is mounted by the strap is achieved by means of a spring clip designed for use with the mounting strap to ensure continuity of electrical connection between the strap and metal housing.

A [72] Inventor:

United States Patent Schumacher [54] GROUNDING DEVICE FOR MOUNTING STRAPWalter C. Schumacher, Warwick, R1. [73] Assignee: General ElectricCompany [22] Filed: April 30, 1971 21 Appl. No.: 138,986

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339/122 R, 132 R, 133 R, 134, 256 R, 258 R; 24/73 B, 73 MF, 8113, 81 BM,84 B, 129 8; 248/215, 301,

[56] References Cited UNITED STATES PATENTS 454,111 6/1891 Coupe..248/493 1,616,957 2/1927 Honigbaumm, ..248/301X [451 a July 25, 1972Primary Examim-rLurumie E. Askin Attorney-Paul E. Rochford, Frank L.Neuhuuser, Oscar B. Waddcll and Joseph B. Forman [57] ABSTRACT Groundingconnection between the mounting strap of a wiring device and the metalhousing in which the device is mounted by the strap is achieved by meansof a spring clip designed for use with the mounting strap to ensurecontinuity of electrical connection between the strap and metal housing.

1 Claim, 5 Drawing Figures PATENTEDJuL 25 m2 FIGS fm/enfor ale/fer 6Sc/zumac/rer GROUNDING DEVICE FOR MOUNTING STRAP The present inventionrelates to improved means for grounding connections between mountingstrap of wiring devices and the wall boxes into which they are mounted.

Conventionally wiring devices are mounted in metal wall boxes and thelike by means of metal screws which extend through openings provided ateach end of a mounting strap to receiving threaded openings at the topand bottom or end of a metal wall box into which the wiring device is tobe placed. It has been found, after a face plate is mounted on thewiring device and a permanent mounting completed at a wall location,that the mounting screw does not always have the desired contact withthe mounting strap or bracket and accordingly that the grounding of thebracket through the screw to the wall box does not necessarily occur.The difficulty may be the result, for example, of the even spacing ofthe screw in the oversize opening at the end of the mounting bracket dueto the realignment of the bracket and the screw supporting it as theface plate is fastened into place. The face plate is conventionallymounted onto the wiring device by means of screws which pass throughappropriate holes in the face plate and attach to the mounting strap ofthe wiring device. A support for the device and its strap relative tothe wall box may result as the edges of the face plate press against thewall to minimize the electrical contact between the mounting screw andthe bracket.

Normally the mounting strap of a wiring device is formed of steel andwhere metal conduit or metal sheath enclosed wire is employed themounting strap is grounded through the metal box and metal sheath ormetal conduit. Alternatively. where the cable used has a grounding wirein the cable, connection of this wire directly to the box and groundingstrap is performed during installation of the wall box. The problem ofbtaining an adequate ground is accentuated where a grounding convenienceoutlet is to be mounted in a flush mounted metal box. In such case thegroundingof the grounding prong of a three prong cap is to be madethrough the metal strap of the convenience outlet and accordingly mustbe in turn made through the mounting screw by which the mounting strapis affixed to the wall box.

According to the National Electric Code, Article 250-74, the groundingof grounding type receptacles by grounding the mounting strap to the boxwith the mounting screws is not acceptable, except when the box in whichthe device is mounted is itself surface mounted, and except when contactdevices or yokes designed and approved for this purpose are employed inan installation which is not surface mounted.

Except in the above cases, the National Electric Code requirement isthat an independent connection as by means of a jumper wire or pigtailbe made between the terminal screw of the grounding strap and the wallbox itself.

Further, according to Article 370- of the National Electric Coderelating to boxes and fittings, it is required that in a wall or ceilingmounting the outlet box may be set back a maximum of one-quarter inch Ainch) from the finished surface of the mounting where the material ofthe wall or ceiling is a non-combustible material. For combustiblematerials, the box must be flush or extend beyond the finished surface.The above described mountings of metal boxes to receive wiring devicesis the wiring scheme most commonly used and requires separate bonding atthe terminal grounding screw of the device to the metal box or the useof an approved contact device or yoke.

It is accordingly one object of the present invention to provide a meansfor ensuring a grounding connection between a mounting strap and agrounded wiring device receiving box.

Additional objects and advantages of the present invention will be inpart pointed out and in part apparent from the description whichfollows.

In one of its broader aspects the objects of the present invention areachieved by providing a conductive metal strip of spring metal, saidstrip having a configuration of slits in the metal to give the strip a Wconfiguration of contact fingers,

being bent at right angle to the plane of the strip into a hook to claspan insulating wall, outer fingers of the W, and the center finger beingbent out for spring contact with the receiving wall box.

The manner in which the present invention may be carried into effectwill be clear from the description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. I is an exploded perspective view of a duplex receptacle equippedwith the grounding clip of the present invention.

FIG. 2 is a perspective view of details of a clip as provided pursuantto the present invention.

FIG. 3 is a sectional view of the end of the receptacle showing the clipin place mounted proximate the grounding strap of the receptacle.

FIG. 4 is a less detailed side elevational view, in part in section, ofthe receptacle as shown in FIG. 3 illustrating a receptacle equipped ateach end with the clip of the present invention.

FIG. 5 is a side elevation, partly in section, showing the duplexreceptacle equipped with the clip of the present invention in place in awall box.

Referring now first to FIG. 1 a duplex receptacle 12 is shown positionedfor mounting into a wall box 10 shown in phantom. Mounting screws 14 arepoised to be threaded in the conventional manner through the screwopening I6 in the receptacle mounting strap 18 and into the threadedtabs 20 formed integrally with the wall box 10 by folding metal into theposition shown in FIG. I. A grounding clip 22 as provided by thisinvention is shown at the upper portion of duplex outlet 12 mountedbehind the mounting strap 18 and disposed against the insulating body 24of the duplex receptacle I2.

Turning now to FIG. 2 a detailed view of the grounding clip 22 is shownwith the clip turned at a angle from the position as shown in FIG. I.The clip is formed from a flat sheet or strip stock and, as is evidentfrom FIG. 2, has a general W form with three upwardly extending fingers26, 28, and 30 arising from a common base 32. The two end or outerfingers 26 and 28 are generally in the plane of the strip together withthe base 32. The center 30 is bent back out of the plane of the base andend fingers. The center finger 30 may have a lower section such as 31,best seen in FIG. 3, bent at a larger angle, and an upper section bentat a shallower angle to the plane of the end fingers and base of theclip. Strain relief openings 34 and 35 may be provided at the ends ofthe slits of the W, Le, the two vertical slits which form the threefingers from the strip. These strain relief openings are disposed sothat the movement of the fingers is freer and so that cracking does notdevelop due to a flexing of finger 30.

The upper ends 36 and 38 of outer fingers 26 and 28 are bent forwardfrom the plane of the fingers 26 and 28 at about right angles to theplane in which the upwardly extending fingers lie. The terminal portionsof the forwardly extending end portions 36 and 38 are bent again atright angles to the end sections to form the holding tabs 37 and 39.

Referring now to FIG. 3 the function of the several parts of the tabwill be made clearer. The numbering of portions of the grounding clipcorresponds to the numbering employed in FIG. 2 so that like numberedparts designate parts described with reference to FIG. 2. Base 32 isseen to be disposed against an insulating wall section 24 of receptacleI2 seen in section in FIG. 3 and 4 and seen as an outer casing surfacein FIG. 1. The upwardly extending finger 26 of the clip also liesagainst the insulating casing. The end section 36 can be seen to becrimped along with bracket 40 between the upper and lower insulating endwall portions 24 and 25 at the end of the receptacle 12. The holding tab37 is shown extending around and into contact with the inside surface ofthe insulating wall section 24. Thus the clip is locked by the clampingof its end sections in the insulating housing of receptacle 12 againstmovement away from the wall section. The mounting strap 40 extends fromthe interior portion of the housing of receptacle 12 through theconforming opening between wall sections 24 and 25. In its positionbetween these sections it is held in direct grounding contact with endsections 36 and 38 of the grounding clip 22. The portion 42 of themounting strap extending from the housing of device 12 extends upwardwith its end portion 44 parallel the portion of the housing.

Referring now to FIG. 4 the position of the grounding clip 22 relativeto the larger portion of the receptacle is shown. The numbers used inFIG. 4 correspond to those described with reference to FIGS. 2 and 3 andcorrespondingly numbered parts have correspondingly described functions.

As illustration of the ultimate use of the receptacle with the clip inplace is seen in H0. 5 wherein the metal box has mounted therein thereceptacle 12. The screws 14 extend through the screw holes 16 andthreaded screw receptacles 20 of the mounting strap ends 18. The springarm 30 of the clip is seen to be held in contact with the metal box wall11.

What is claimed is:

l. A self grounding electrical receptacle said receptacle having aninsulating housing comprising an two right angle bends to form hooksextending in one direction out of the plane of said strip, the middlefinger being bent from its point of attachment to the base in anopposite direction out of the plane of the strip, said outer fingersbeing mounted against the casing wall, and the hooks being sandwiched ingrounding contact with said strap between said casing and cover.

I i k

1. A self grounding electrical receptacle said receptacle having an insulating housing comprising an insulating casing and an insulating cover and a mounting strap sandwiched between the casing and cover, a grounding clip to ensure grounding of said strap to a wall box said clip comprising a strip of conductive spring metal, said strip having two slits extending inward from a common edge to form a W configuration of conductive fingers rising from a common base, the outer fingers of said W having the ends thereof bent with two right angle bends to form hooks extending in one direction out of the plane of said strip, the middle finger being bent from its point of attachment to the base in an opposite direction out of the plane of the strip, said outer fingers being mounted against the casing wall, and the hooks being sandwiched in grounding contact with said strap between said casing and cover. 